Forging is a process by which the shape and physical properties of metal can be changed. The process involves placing the piece of metal (normally heated) between the halves of a die and forcing the die to close by impact or pressure. The operation causes a controlled plastic deformation of the metal into the cavities of the die. This flow of material results not only in a change in the shape of the metal, but also improves the uniformity of the metal, improves its grain structure, and causes a shape-conforming grain flow. The resulting workpiece has properties which are superior to those generated by other methods, thus making forging essential where high performance workpieces are required.
One of the difficulties in the forging process is that, when a complex shape is to be manufactured, the workpiece which is fed to the final forging die must be relatively close to the final desired shape. This workpiece (which is shaped properly to be placed in the final forging process) is called a "preform." The design and formation of this preform can be critical to the economics of the forging process, since these factors affect the useful life of the final forging dies and the power requirements of the press and the amount of waste of metal in the flash.
In the past, two general procedures were used to develop the preforms. Where maximum performance of the workpieces is required, a series of forging operations can be used. The standard mill stock may be exposed to several roller impressions, some bending and twisting impressions, gathering and conditioning impressions, and then blocking impressions. Each of these operations requires substantial expenditures for specialized forming equipment and the cost of the operations can only be justified in extreme situations. In workpieces in which the strength requirements are not as rigorous, the preform can be machined from the mill stock using standard cutting methods. This process can involve a great deal of wasted material and, its practicality is severely effected by the spiraling increase in the cost of the various resources necessary to carry it out.
It has been suggested that the shortcomings of the above processes could be eliminated in certain situations by the formation of forging preforms from powdered metal. When a powdered metal is forged, its porosity is substantially reduced and its mechanical properties may be improved, so that they are equal or close to those obtained in forged conventional metal. While the arguments in favor of such a process make a great deal of sense, the practical problems of carrying out an economic system (particularly where large or complex shapes are involved) have not been satisfactorily solved. If powder metallurgical preforms are made in "hard" tooling, the presence of multi-level portions in the pressing direction, the presence of side wall friction, and the presence of tall parts all pose severe problems for their manufacture. When the preform is made with "soft" tooling, as used in isopressing, the need for time-consuming bag filling, supporting, bag stripping, and pressurizing and de-pressurizing cycles tend to reduce production rates to unacceptably low levels. These and other difficulties experienced with the prior art processes have been obviated in a novel manner by the present invention.
It is, therefore, an outstanding object of the present invention to provide an efficient and economically advantageous process for forming forged articles.
Another object of this invention is the provision of a process in which the preforms of the forging process are generated from powdered metal.
A further object of the present invention is the provision of a forging process in which the preforms are formed by casting powdered metal in semi-rigid molds, isostatically compacting the mold and its contents, removing the mold from the compacted contents, and sintering the green contents to generate the preform.
It is another object of the instant invention to provide a forging process in which a powdered metal preform is developed in a method which can be adapted to automation, because the steps involve relatively simple mechanical operations which can be carried out in a manner consistent with modern manufacturing techniques.
A still further object of the invention is the provision of a forging process which results in forged articles which have mechanical properties equal or close to articles manufactured by conventional methods.
With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of steps set forth in the specification and covered by claims appended hereto.